The advantages of amperometric biosensors which incorporate an enzyme as a biocatalyst have been reviewed in some detail by Aston and Turner, (1984) Biotech. Genet. Eng. Rev. (ed. G. Russell), 1, 89-120, Intercept, Newcastle-upon-Tyne, and by Davis G., (1985) Biosensors, 1, 161-178. They vary in the mode of signal transduction, and different types may be loosely classified as (a) those in which the electrical response arises from the oxidation of a product of the enzyme reaction at an electrode; (b) "mediator assisted", in which electrons are transported from the enzyme to the electrode with the aid of an oxidation-reduction ("redox") reagent, or (c), "direct electron transfer" (DET), in which no such mediator assistance is required.